Dynamic clothing animated draping question

Hello all!

I'm working on an image using some dynamic clothing, and I'm having an issue with the draping. My figure is lying down, and I've read in the past that the best way to handle the draping on a pose such as this is with an animated drape - moving figure into position, as well as moving the "floor" (in this case, a plane) up underneath the figure so that the clothes will "lie" on the floor.

So, I've started off with the default pose. The pants are set to collide with themselves, with everything on the figure from her abdomen on down, and with the plane. The plane is starting off lowered a bit beneath her. (first image)

I then created a keyframe at 20 seconds with her final pose, and moved the plane up underneath her. (second image) Finally, I let the animation go for 10 more seconds, so that by 30 seconds the pants would have settled to the floor. (third image)

However, when I do animation, her pants still poke through the floor - which is exactly what I DON'T want to happen. (fourth image)

Anyone have any ideas why this is happening, and how to fix it?

Thanks in advance!

Michael